When feeding AC power through long cables in transmission systems feeding an electrical load (machines/or other loads), the voltage at the consumer end will be heavily influenced by the electrical load drawn by the consumer.
An example of such a long cable is a cable some kilometers in length connecting a power supply comprising a frequency converter on land to a transformer or to a motor of a pump installed out to sea. In the field of Oil and Gas exploration and production, such loads or machines providing long step-out applications may for example be mounted on the seabed tens of kilometers away from land or away from a topside platform or land installation comprising the frequency converter. Electrical equipment such as a subsea multiphase pump or pressure booster pump or a subsea compressor used in Oil and Gas production or transfer installations may be operated underwater, e.g. on the seabed, at depths 1000 meters, or more.
For electrical consumers like induction motors, the effect of long power cables can generate over voltages caused by self excitation. Self excitation of induction motors can occur if the electrical source is suddenly disconnected during operation, or power flow from supply is obstructed, for instance like stopping the modulation of a frequency converter. This phenomenon occurs in situations where the reactive power production on the cable reaches a higher level than the reactive power consumed by the electric machine. The capacitive contribution from the cable may therefore cause an induction motor to enter self excitation, which situation can lead to harmful over voltages.
Therefore, in these long step-out applications, with long cable lengths, there is a potential risk for over voltage generation due to self excitation.
In order to avoid the connected electrical equipment being harmed it is then important to protect it from these overvoltages.
One known protective circuit comprises the diode bridge.
U.S. Pat. No. 7,227,325 does for instance describe a protective circuit with a diode bridge and thyristor as switch used for protecting a frequency converter. The diode bridge is placed close to a motor and here short circuits the connection between the frequency converter and the motor in response to the detection of a fault in the frequency converter.
U.S. Pat. No. 7,164,562 describes a protection configuration provided for short-circuiting a converter in case of network faults. The protection circuit is here connected to the rotor of a generator.
Various other protective circuits are described in U.S. Pat. No. 5,734,256, U.S. Pat. No. 4,766,360, U.S. Pat. No. 6,226,162 and U.S. Pat. No. 7,479,756.
However none of the documents describe the problem of protecting against over-voltages due to self-excitation caused by the combination of long cables and electric machines.